With development of high density mounting technology, severer requirements such as high withstand voltage have been imposed on printed circuit boards on which electronic devices are mounted in a high density. These kind of printed circuit boards are formed by superposing a plurality of (e.g. seven) cloths, woven of glass fibers with paste, interposing epoxy resin between them.
If a high voltage is applied to printed circuit boards made of glass fibers containing minute metal pieces, discharges occur due to the minute metal pieces, so that it is impossible to get printed circuit boards with high withstand voltage if they use glass fibers containing minute metal pieces. Therefore it is necessary to use glass fibers without minute metal pieces for manufacturing high quality printed circuit boards. The development of technology to detect minute metal pieces contained in glass fibers has been a longstanding need in this technical field. The applicant of the present application already proposed in JP Pat. Kokai No. 63-145951 a microwave cavity resonator in which a protrusion is arranged in the center so that physical quantities of stringy materials can be precisely measured. As shown in FIGS. 5A and B, the cylindrical microwave cavity resonator 1 comprises protrusion 4 on its center and space 3 for passing stringy material 2 arranged on the center of protrusion 4 and the wall of the microwave cavity resonator opposed to the protrusion.
Such microwave cavity resonator 1 allows to measure metals contained in stringy materials such as glass fibers. To do so glass fibers without metal pieces are first placed at space 3 and then resonance characteristic for the glass fibers are measured with a measuring instrument of resonance characteristic by scanning the frequencies of microwave supplied to the microwave cavity resonator 1 from antenna 5, which frequencies are changed by changing the voltage of the voltage control oscillator. Next if glass fibers containing metal pieces are placed at space 3 for measuring resonance characteristics, the resonance characteristics will be different from that of the glass fibers without metal pieces. Measuring of shifted amount in resonant frequency and decreased amount in resonance peak voltage enables not only to detect metals contained but also to measure the accurate amount of contained metal.
The method of detecting metals by the change of resonance characteristics in this manner has an advantage to obtain accurate measurement results, but it has a disadvantage that the total system costs too much because it needs a voltage control oscillator and a measuring instrument of the resonance characteristics. In case of glass fibers by which printed circuit boards for high density assembling are manufactured, measuring the amount of contained metals is not necessary but only detection of contained metals is necessary. Furthermore in factories hundreds of, glass fibers are manufactured at the same time and a detecting apparatus for contained metals is needed for every glass fiber, so that the total cost of the detecting apparatus would be enormous. Therefore an acceptable detecting apparatus of contained metals in a stringy material must be inexpensive. In this sense the measuring apparatus for physical quantities disclosed in Pat. Kokai No. 63-145951 has some disadvantage when the introduction of the apparatus to factories of manufacturing glass fibers is considered.
On the other hand, in case of glass fibers used for high density printed circuit boards the only problem is whether or not metals are contained in the glass fibers. From this point of view, the detecting apparatus of physical quantities, as disclosed in Pat. Kokai No. 63-145951, which can measure not only the existence of metals but also accurately measure the amount of contained metals is an unnecessarily accurate detecting apparatus for use when manufacturing glass fibers.
One of objects of the present invention is to provide an inexpensive detecting apparatus of conductive materials which can accurately detect the existence of minute metal pieces contained the material to be measured such as stringy materials.
The present invention solves the above mentioned problem by providing a detecting apparatus of conductive materials contained in a stringy material comprising a cylindrical microwave cavity resonator having a circumferential portion and disks covering the both ends of the circumferential portion, which comprises a cylindrical protrusion arranged on the center of at least one of the disks, at least one antenna provided on the circumferential portion, a detecting means for discharge, and a pass for the stringy material to be measured arranged in high electric field region formed by the protrusion and penetrating the cylindrical microwave cavity resonator.
When detecting of conductive materials such as minute metal pieces contained in stringy materials with the detecting apparatus of conductive materials of the invention, the microwave cavity resonator is made to resonate at a constant frequency by microwaves supplied from a discharge generating antenna. The microwave cavity resonator of the detecting apparatus of the invention comprises at least one protrusion at its center where the electric field is enhanced. The pass for the stringy materials to be measured penetrating the cylindrical microwave cavity resonator is arranged at a place where the the electric field is the highest. If minute metal pieces contained in the stringy materials locate at such place, the electric fields concentrate to the minute metal pieces, so that discharge will occur. At this time, since lights, sound and electromagnetic waves are generated, discharges can be detected by a discharge detecting means such as a photodiode, a microphone, or a loop antenna. In this way metal pieces contained in the stringy materials can be sharply detected.
The only required function for the microwave generator used in the apparatus of the invention is to generate a discharge, so that it does not need an expensive voltage control oscillator which is essential for the prior method for measuring shift in resonance characteristic. Inexpensive magnetrons which are commonly used in consumer electric ranges are suitable enough for generators used in the system of the invention. In addition expensive measuring circuit for resonance characteristic, which is necessary for the prior measuring method, is not needed. Therefore the detecting apparatus of the invention is a very inexpensive apparatus which can be employed in a large quantity in factories of manufacturing glass fibers.
The apparatus characterized in that the two protrusions are opposed each other can supply larger electric fields to the stringy materials to be measured than the apparatus with only one protrusion.
In case of the apparatus characterized in that the pass is a vertical notch extending from the center axis to the circumferential portion, it is easy to set stringy materials to be measured on said apparatus because a user only need to move the stringy material to be measured from its circumferential portion to the center. In addition very few microwaves leak from said apparatus at the time of measurement, so that it is not necessary to cover the notch portion with dielectric after setting the stringy materials to be measured on the pass.
The apparatus characterized in that the pass is a vertical notch extending from the center axis to the circumferential portion has an advantage that setting stringy materials to be measured on said apparatus is easy. The apparatus characterized in that the pass is a penetrated hole including the center axis of the microwave cavity resonator and the apparatus characterized in that apertures are arranged on the circumferential portion where a straight line perpendicular to the center axis of the microwave cavity resonator hits, have both an advantage that microwaves leaked from the microwave cavity resonator at the time of measurement are fewer than ones in the apparatus without said notch.